1. Field of the Invention
The present invention relates to an electronic endoscope, and more specifically relates to a light source apparatus for an electronic endoscope using a plurality of aperture controlling rotary plates.
2. Description of the Prior Art
In conventional electronic endoscopes, in order to provide appropriate light modulation, an endoscope recording apparatus has been proposed in, for example, U.S. Pat. No. 4,729,018 (based on Japanese Unexamined Patent Publication No. 62-69222). The apparatus disclosed in this publication is provided with a rotary shutter having a rotatable shaft, wherein the distance between the axis of the rotatable shaft and the optical axis of illuminating light emitted from a light source for the endoscope is variable. The rotary shutter is shaped so that a difference in peripheral speed occurs between the radial portions thereof, i.e., the rotary shutter is shaped so that the aperture size varies during rotation of the rotary shutter. Light modulation is carried out by altering the distance between the axis of the rotatable shaft and the optical axis of illuminating light and by utilizing the peripheral speed difference of the rotary shutter.
In Japanese Unexamined Patent Publication No. 62-69222, light modulation can be performed, however, the structure of the rotary shutter is complex. Furthermore, it is necessary to provide a mechanism to vary the distance between the rotary shutter and the optical axis of the light source for the endoscope. Accordingly, the manufacturing cost is high and the manufacturing process is troublesome. Moreover, in order to achieve such a construction, the outer diameter of the rotary shutter must be several times larger than that of the light bundle, thus resulting in an increase in size of the rotary shutter. If the rotary shutter is asymmetrical in shape with respect to the rotation axis thereof in order to vary the aperture, the center of rotation does not align with the center of gravity, so that the rotary shutter tends to lose balance during rotation. Consequently, correct emission of the illuminating light cannot occur, and the rotary shutter and the surrounding members may break.
Upon considering the above-mentioned problems, the applicant of the present invention has proposed a light source apparatus for an electronic endoscope including a light source; a rotary shutter for intercepting or transmitting light emitted from the light source toward a light guide, the rotary shutter including a pair of aperture controlling rotary plates having axes coaxial with each other and extending parallel with an optical axis of the light source. The aperture controlling rotary plates can be rotated together (integrally) with each other and each includes light interception portions and opening portions alternately arranged in a rotation direction, wherein a combined opening angle of the opening portions of the rotary shutter is varied by a relative rotation of the pair of aperture controlling rotary plates (Japanese Unexamined Patent Publication No. 2006-102474).
The invention disclosed in Japanese Patent Application No. 2005-26568 discloses a light source apparatus for an electronic endoscope including a pair of planetary gear mechanisms which are driven by a phase difference motor, provided separate from a chopper motor, and furthermore, the pair of planetary gear mechanisms are associatively connected to a pair of aperture controlling rotary plates, respectively. Accordingly, when only the chopper motor is driven, the pair of aperture controlling rotary plates rotate at the same rotational speed so that the aperture angle of the aperture remains unchanged. On the other hand, if both the chopper motor and the phase difference motor are driven, a difference in rotational speed between the pair of aperture controlling rotary plates occurs, so that the pair of aperture controlling rotary plates rotate while changing the aperture angle of the aperture.
In the invention disclosed in Japanese Patent Application No. 2005-26568, since the output gear of the phase difference motor and the input gear of the planetary gear mechanism which corresponds to the phase-difference motor are in mesh with each other, backlash occurs between the output gear and the input gear. When such backlash occurs, the rotational control of the pair of aperture controlling rotary plates cannot be correctly carried out.
Furthermore, the above described problem also occurs in the same manner when the rotational force of the phase difference motor is reduced in speed or increased in speed in the case where a gear mechanism having a plurality of gears is provided between the phase difference motor and the above-mentioned output gear.